1. Technical Field
The present invention relates to a position calculating method and a position calculating device.
2. Related Art
A GPS (global positioning system) is widely known as a positioning system using a positioning signal and is used for a position calculating device built in a mobile phone, a car navigation apparatus, or the like. The GPS performs a position calculation operation for calculating its own position on the basis of the information, such as the positions of a plurality of GPS satellites or a pseudo range from each GPS satellite to the GPS.
As a known example of the position calculation operation, calculation using a least square method is widely used. A technique of calculating the position by performing a position calculation operation using a Kalman filter was also proposed as another method (for example, JP-A-2001-337156).
The technique using the Kalman filter is an estimation method based on the probability theory of estimating the state function changing with time by using an observed value including an error. The Kalman Filter method includes an estimation process to estimate a state function of a target system and a correction process to correct the estimated state function. The state function is generally corrected and updated with given externally-observed values, but in this method the expected observed-value is given as an observation error. An observation error is generally known as a value R.
The position calculation using the Kalman filter generally uses a position or a moving speed of a position calculating device as a state function and externally-observed values such as a phase or a frequency of GPS satellite signals received by the position calculating device as observed values to apply to the positioning calculation logic of the Kalman Filter. However, since the magnitude of error in the observed values is unknown, the calculation is often performed using a predetermined fixed value in place of the observation error.
If the position is calculated using a larger observation error than the actual error in the observed values, the filter may effect so strongly that a “position delay”, in which the calculated position is temporally delayed with respect to the actual position, or a “shortcut”, in which the followability of the calculated position lowers at curves or turns, may occur. As a result, accuracy in the position calculation lowers.